1. Field of the Invention
This invention pertains to dimmer circuits for high intensity, gaseous discharge (HID) lamps and more particularly to such a dimmer that provides dimming current to the lamp through at least partial bypass of a reactive ballast as determined by the positioning of unipolar dc pulses applied during a permissible time range of applied line voltage.
2. Description of the Prior Art
U.S. Pat. No. 3,816,794, Snyder, describes a circuit employing a two-part reactive ballast connected in series with a high intensity, gaseous discharge lamp. One of the two elements of the ballast is connected across the main terminals of a triac operating as a gated bypass means. When the triac conducts, a current path is established through the triac, at least partially bypassing the reactive element. The duration of conduction determines the total amount of current through the ballast, and hence through the lamp, thereby providing a means for establishing the brightness of the lamp.
In the circuit described in U.S. Pat. No. 3,816,794, low gate source or drive voltage to the gate of the gated bypass triac is derived from a potentiometer, an isolating transformer circuit, a second triac and a Zener diode network, together with other components. The gated bypass triac is fired from a gate source in phase with line voltage, the amplitude being controlled by a gate-signal control device including a Zener diode to properly time the turning on of the triac in relation to lamp current. The Zener diode also prevents the triac from being triggered past a time when there might be opposite polarity ballast-element voltage and lamp current, which would cause flicker of the lamp.
U.S. Pat. No. 3,894,265 discloses a circuit that provides a control network for the gated bypass network including the programmable unijunction transistor. Ready connection to single power and three-phase power systems is achieved, but the gating of the bypass triac is by ac gating.
Variations in controlling the timing operations to a gated semiconductor connected for at least partial current bypass operation of a ballast connected to an HID lamp are shown in the following patent applications: patent application Ser. No. 927,555, "Optocoupler Dimmer Circuit for High Intensity Gaseous Discharge Lamp", filed July 24, 1978; patent application Ser. No. 930,913, "High Frequency Dimmer Circuit for High Intensity Gaseous Discharge Lamp Dimmer", filed Aug. 4, 1978; patent application Ser. No. 936,883, "Non-Interfering, Overlapping High Frequency Signalling for Lamp Dimming Circuit", filed Aug. 25, 1978; and patent application Ser. No. 936,882, "Variable Low Frequency Dimming for High Intensity Gaseous Discharge Lamps", filed Aug. 25, 1978, all of the same inventor as the present application.
Although there are many schemes of gating and developing gating signals, none show the use of twin timers for developing a readily movable and variable width dc pulse for gating within a time range of ac line voltage.
It is therefore a feature of the present invention to provide an improved dimmer having these desirable features and further to provide an improved pulsing circuit whose width is dependent on the forward advancement within a time range of an associated ac voltage.